1) Field of the Invention
The present invention relates to a multicast transmission method of transmitting data to a plurality of nodes (computers, terminals) belonging to a specific group in a communication network based upon a protocol such as TCP/IP (Transmission Control Protocol/Internet Protocol).
2) Description of the Related Art
In recent years, with the progress of distributed processing computer networks, a need to simultaneously send the same data to a plurality of nodes has increased for the purpose of improving the processing efficiency. In the case of 1-to-N (one-to-many) or N-to-N communications (for example, for the execution of an application such as a television conference at a plurality of nodes), such a situation as to transmit the same data to a plurality of nodes grows as compared with one-to-one communications. For coping with such a situation, computer networks employ a multicast function to concurrently forward data a plurality of modes pertaining to a specific group.
In general, the Ethernet has been known as the typical protocol for the LAN (Local Area Network) constituting a computer network. In this Ethernet, for realizing the aforesaid multicast function, a group address (multicast address) is first set and registered in a communication driver of each of the nodes pertaining to a specific group (multicast group). Subsequently, a node, which transmits data, broadcasts transmission data including an address designating the specific group toward all the nodes except the data transmitting node. The communication driver of each of the nodes receiving the data decides whether or not the address given to the transmitted data coincides with an address assigned previously to its own terminal. If the coincidence between the both addresses takes place, the communication driver accepts that data, whereas, if no coincidence occurs, it abandons the same data.
In addition, Japanese Patent Laid-Open (Kokai) No. Hei 5-324545 discloses a technique (a bus control system for easily realizing the initial connection procedure in broadcasting) to designate a receive terminal group to which data should be transmitted. In the technique disclosed in this publication, a group address signal line is provided in buses for establishing connections among all the terminals (nodes), while a group address representative of a group each of the terminals belongs to is assigned in advance to that terminal. Further, in transmitting data to only a specific group, a transmission terminal first broadcasts the group address designating the specific group through the group address signal lines to all the terminal other than this transmission terminal. Each of the receive terminals receiving the group address checks whether or not the group address transmitted coincides with its own group address set in advance. The receive terminal, whose group address agrees with the transmitted group address, goes into a receive ready condition. Following this, the transmission terminal side starts to send (broadcast) data to the receive terminal belonging to the specific group.
However, with the multicast transmission method on the Ethernet, the data to be transmitted to the nodes belonging to a specific group is broadcasted in a state where a group address is given thereto, along with the imposition of a burden on the processing on the transmission side or on the reception side, the network or nodes having no relation also undergo its influence, and the traffic in the whole communication network increases.
The multicast communication targets the multimedia including moving picture data. The moving picture data commonly requires the transmission of 30 scenes (frames) per second, which produces an extremely large volume of data. Accordingly, in the case of a large volume of data such as such moving picture data or voice data being transmitted according to the above-mentioned multicast communication method, a significant increase in the traffic in the whole communication network takes place.
Moreover, the technique disclosed in Japanese Patent Laid-Open (Kokai) No. Hei 5-324545 is designed such that a transmission terminal designates a receive terminal belonging to a specific group to make it go into a receive ready condition before the transmission terminal side broadcasts data and the terminal being in the receive ready condition (that is, the receive terminal pertaining to the specific group) receives the broadcasted data. However, since the transmission from the transmission terminal to the receive terminal fully depends upon the broadcasting, the traffic on the communication network increases, and the transmission of a large volume of data such as moving picture data and voice data according to the aforesaid technique causes a remarkable increase in the traffic on the communication network.